ndogenous ecotropic viruses are known to be differentially xpressed in various inbred mouse strains. AKR mice, which may arbor up to five copies of such endogenous ecotropic proviral NAs, produce virus shortly after birth and most animals evelop leukemia within a year. In contrast, BALB/c mice have nly a single endogenous ecotropic provirus and only a few nimals (10-15%) develop leukemia late in life. To evaluate hy BALB/c mice inefficiently express ecotropic proviruses, we loned and sequenced the LTR of one of the BALB/c endogenous rovirus and compared it to the LTR of inducible ecotropic roviruses isolated from AKR mice. Nucleotide sequence nalysis revealed two base differences in the U3 region of the wo LTRs. o study the relationship between dual-tropic MuLVs and eukemogenesis, we have cloned two dual-tropic MuLVs, MCF-13 thymotropic strain) and MCF-111 (non-thymotropic strain), from hronically infected mink cells. Three MCF-13 clones were solated. ransfection of cloned DNAs onto minks cells resulted in the roduction of infectious progeny virions. The cloning of these wo types of recombinant MCF MuLVs will permit the molecular issection of viral genes in experiments evaluating hymotropism, mink cell focus formation, and leukemogenesis. hese cloned DNAs will be studied in combination with other elevant MuLV clones such as ecotropic, xenotropic, and ndogenous proviral DNAs previously isolated by us from nfected or normal mouse cells.